1. Field of the Invention
The present invention relates to a measurement apparatus, such as a surface plasmon resonance measurement apparatus, for obtaining the physical properties of a sample by utilizing generation of surface plasmons.
2. Description of the Related Art
Free electrons collectively oscillate in metal, and compressional waves called plasma waves are generated. When the compressional waves generated on the surface of the metal are quantized, they are called surface plasmons.
Conventionally, various surface plasmon resonance measurement apparatuses have been proposed for carrying out quantitative analyses on a substance in a sample, by utilizing a phenomenon where the surface plasmons are excited by a light wave. Among the surface plasmon resonance measurement apparatuses, an apparatus using a system called the Kretschmann configuration is well known (as disclosed in, for example, Japanese Unexamined Patent Publication No. 6 (1994)-167443).
A surface plasmon resonance measurement apparatus using the above-mentioned system basically includes a dielectric block which is, for example, prism-shaped, a metal film which is formed on a face of the dielectric block and brought into contact with a sample, and which has a refractive index lower than that of the dielectric block, and a light source for generating a light beam. The surface plasmon resonance measurement apparatus also includes an incident optical system for causing the light beam to enter the dielectric block at an angle which satisfies total reflection conditions at the interface between the dielectric block and the metal film. The surface plasmon resonance measurement apparatus also includes a light detection means for detecting a surface plasmon resonance state, namely an attenuated total reflection state, by measuring the intensity of the light beam which is totally reflected at the interface.
A relatively thin light beam may be deflected and caused to enter the interface so that the light beam enters the interface at various incident angles as described above. Alternatively, a relatively thick light beam in a convergent state or divergent state may be caused to enter the interface so that the light beam includes components that enter the interface at various angles. In the former case, as the light beam deflects, the reflection angle of the light beam changes. The light beam may be detected by a small light detector which moves synchronously with the deflection of the light beam. Alternatively, the light beam may be detected by an area sensor which extends along the direction of a change in the reflection angles. Meanwhile, in the latter case, light beams may be detected by an area sensor which extends in a direction so that it can detect each of all the light beams reflected at various reflection angles.
In the surface plasmon resonance measurement apparatus configured as described above, when a light beam is caused to enter the metal film at a specific incident angle θsp which is larger than or equal to a total reflection angle, an evanescent wave is generated. The electric field of the evanescent wave is distributed in the sample which is in contact with the metal film. Then, surface plasmons are excited at the interface between the metal film and the sample by the evanescent wave. When wave number matching is achieved as the wave number vector of the evanescent light is equal to the wave number of the surface plasmon, the evanescent light and the surface plasmon resonate. Then, light energy is transferred into surface plasmons. Therefore, the intensity of the light which is totally reflected at the interface between the dielectric block and the metal film sharply drops as illustrated in FIG. 3. Generally, the drop in the intensity of the light is detected as a dark line by the light detection means.
The resonance as described above only occurs when the incident beam is p-polarized. Therefore, it is required to set the surface plasmon resonance measurement apparatus in advance so that the light beam enters the interface in p polarization.
If the wave number of the surface plasmon is obtained based on an incident angle θsp when attenuated total reflection (ATR) occurs, the dielectric constant of the sample can be obtained. Specifically, if the wave number of the surface plasmon is Ksp, an angular frequency of the surface plasmon is ω, the speed of light in a vacuum is c, and the dielectric constants of the metal and the sample are ∈m and ∈s, respectively, the following relationship is satisfied:
            k      sp        ⁡          (      ω      )        =            ω      c        ⁢                                                      ɛ              m                        ⁡                          (              ω              )                                ⁢                      ɛ            s                                                              ɛ              m                        ⁡                          (              ω              )                                +                      ɛ            s                              
If the dielectric constant ∈s of the sample is known, the density of a specific substance in the sample can be obtained based on a predetermined calibration curve or the like. Consequently, an incident angle θsp when the intensity of the reflected light drops can be obtained. Accordingly, the dielectric constant of the sample can be obtained. Consequently, the refractive index of the sample and the physical properties corresponding to the refractive index can be obtained.
Further, when a sensing material which specifically binds to a specific substance in the sample is fixed onto the metal film, if the specific substance is contained in the sample provided on the metal film, the specific substance binds to the sensing material. Accordingly, the refractive index of the sensing material changes. Therefore, the specific substance can be detected by detecting a change in the refractive index.
Further, a leaky mode measurement apparatus described, for example, in “Surface Refracto-sensor using Evanescent Waves: Principles and Instrumentations”, by Takayuki Okamoto, Spectrum Researches, vol. 47, No. 1, 1998, pp. 21 through 23, 26 and 27 is also known as a similar measurement apparatuses utilizing the attenuated total reflection (ATR). The leaky mode measurement apparatus basically includes a dielectric block which is, for example, prism-shaped, a clad layer formed on a face of the dielectric block, and an optical waveguide layer which is formed on the clad layer and brought into contact with the sample. The leaky mode measurement apparatus also includes a light source for generating a light beam and an optical system for causing the light beam to enter the dielectric block at various angles so that total reflection conditions are satisfied at the interface between the dielectric block and the clad layer, and attenuated total reflection occurs due to excitation of a waveguide mode in the optical waveguide layer. The leaky mode measurement apparatus also includes a light detection means for detecting an excitation state of the waveguide mode by measuring the intensity of the light beam which has been totally reflected at the interface. The excitation state of the waveguide mode is an attenuated total reflection state.
In the leaky mode measurement apparatus configured as described above, when the light beam is caused to enter the clad layer through the dielectric block at an incident angle which is larger than or equal to a total reflection angle, the light beam is transmitted through the clad layer. After the light beam is transmitted through the clad layer, only light which has a specific wave number, and which has entered at a specific incident angle, propagates in a waveguide mode in the optical waveguide layer. When the waveguide mode is excited as described above, most of the incident light is absorbed in the optical waveguide layer. Accordingly, attenuated total reflection, in which the intensity of light totally reflected at the interface sharply drops, occurs. The wave number of the waveguide light depends on the refractive index of the sample on the optical waveguide layer. Therefore, if the specific incident angle when the attenuated total reflection occurs is obtained, the refractive index of the sample and the properties of the sample, which are related to the refractive index, can be measured.
There are various kinds of methods for analyzing samples by measuring the intensity of the light beam totally reflected at the interface using a light detection means. The samples may be analyzed as disclosed in “Porous Gold in Surface Plasmon Resonance Measurement”, by D. V. Noort, et al., EUROSENSORS XIII, 1999, pp. 585-588. In this method, light beams which have a plurality of wavelengths are caused to enter the interface at incident angles which can satisfy total reflection conditions. Then, the intensity of the light beams which are totally reflected at the interface is measured for each of the wavelengths, and the degree of attenuated total reflection is detected for each of the wavelengths.
Alternatively, the samples may be analyzed as disclosed in “Surface Plasmon Resonance Interferometry for Micro-Array Biosensing”, by P. I. Nikitin, et al., EUROSENSORS XIII, 1999, pp. 235-238. In this method, the light beams are caused to enter the interface so that total reflection conditions are satisfied. At the same time, a part of the light beams is separated into a spectrum before they enter the interface, and the spectral light beams are caused to interfere with the light beams which were totally reflected at the interface. Then, the intensity of the light beams after interference may be detected to analyze the samples.
When the physical properties of samples are analyzed, there are cases in which a plurality of samples is required to be measured under the same conditions. There are also cases in which information about the two-dimensional physical properties of the samples is required to be obtained. The surface plasmon resonance measurement apparatus and the leaky mode measurement apparatus, as described above, may be also applied to these cases (please refer to Japanese Unexamined Patent Publication No. 2001-255267 and Japanese Unexamined Patent Publication No. 2001-511249, for example). A case of applying the surface plasmon resonance measurement apparatus will be described as an example. The relationship illustrated in FIG. 3 changes in the direction of the horizontal axis of FIG. 3 as the refractive index of a substance which is present on the metal film changes. Therefore, when a light beam is caused to enter a region which two-dimensionally spreads on the interface, at a predetermined incident angle, if a light component enters a part of the region, which has a refractive index as attenuated total reflection occurs when the light beam enters at the incident angle, the light component is detected as a dark line. Specifically, the part of the region is a region at which a specific substance is present on the metal film. Therefore, if parallel light which has a relatively wide cross-section of beams is used, and the distribution of the intensities of light on the cross-section of the light beams totally reflected at the interface is detected, the distribution of the specific substance within a plane along the interface can be measured. Further, as illustrated in FIG. 3, the intensity of the totally reflected light becomes lower around the predetermined incident angle θsp. Therefore, the distribution of the intensities of light on the cross-section of the light beams which entered the interface at a predetermined incident angle, and which were totally reflected at the interface, shows two-dimensional distribution of the refractive indices of the substance (sample) which is present on the metal film.
When the leaky mode measurement apparatus is used, the attenuated total reflection occurs because of excitation of the waveguide mode in the waveguide layer instead of the surface plasmon resonance. However, other features are the same as the surface plasmon resonance measurement apparatus. Therefore, even if the leaky mode measurement apparatus is used, it is possible to obtain the two-dimensional physical properties of the sample in the same manner as the surface plasmon resonance measurement apparatus.
In the specification of the present application, the phrase “to obtain the two-dimensional physical properties of the sample” refers to obtainment of the two-dimensional physical properties of a single sample. The phrase also refers to obtainment of the physical properties of a plurality of the same kind of samples or various types of samples, which is two-dimensionally arranged on a thin film layer, so that the physical properties of each of the plurality of samples are obtained independently from each other.
In the surface plasmon resonance measurement apparatus and the leaky mode measurement apparatus as described above, the light beams are caused to be totally reflected at the interface between the dielectric block and the thin film layer (the thin film layer is the metal film in the former case, and it is the clad layer and optical waveguide layer in the latter case). Accordingly, evanescent waves which are generated in the total reflection state and the surface plasmon or the waveguide mode are coupled with each other. A similar surface plasmon resonance measurement apparatus and leaky mode measurement apparatus may be configured by forming a diffraction grating on a face of the dielectric block instead of causing the light beams to be totally reflected at the face of the dielectric block. Specifically, in that case, if the light beam is caused to enter the diffraction grating from the side of the dielectric block, evanescent light is generated by diffraction. The evanescent light penetrates into the thin film layer, and is coupled with the surface plasmon or the waveguide mode. Therefore, the intensity of light which is reflectively diffracted toward the dielectric block attenuates. Hence, the refractive index of the sample and the physical properties of the sample, related to the refractive index, can be analyzed by obtaining the incident angle of the light beam which enters the diffraction grating when the intensity of light attenuates.
Further, the surface plasmon resonance measurement apparatus and the leaky mode measurement apparatus as described above are used to analyze samples by utilizing a characteristic that an incident angle θ of the light beam, when totally reflected light or reflectively diffracted light attenuates, changes according to the refractive index of the sample. However, the samples may be analyzed in a similar manner even if the incident angle θ is constant. Specifically, if the incident angle θ of the light beam is constant, totally reflected light or reflectively diffracted light attenuates when the wavelength λ of the light beam is a specific value λsp, as illustrated in FIG. 4. The specific value λsp of the wavelength, when the totally reflected light or the reflectively diffracted light attenuates, is determined by the refractive index of the sample. Therefore, if the specific value λsp of the wavelength is detected, the refractive index of the sample and the physical properties of the sample, related to the refractive index, can be analyzed.
The measurement apparatus as described above is particularly advantageous to obtain the two-dimensional physical properties of the sample. Specifically, when the two-dimensional physical properties of the sample are obtained, a light source which generates light beams, such as white light having a certain range of wavelengths, is used. Further, a two-dimensional light detection means for spectrally detecting the totally reflected light or the reflectively diffracted light is used. Since it is not required to change the incident angle of the light beam which enters the interface or the diffraction grating, a predetermined position of the sample can be stably irradiated.
In the surface plasmon resonance measurement apparatus and the leaky mode measurement apparatus as described above, a two-dimensional image detected by the two-dimensional light detection means may be distorted. The aspect ratio of the two-dimensional image may be different from that of an image produced on an actual measurement plane. Specifically, the actual measurement plane is a face of the dielectric block, on which a thin film layer (the thin film layer is a metal film in the case of the surface plasmon resonance measurement apparatus, and it is a clad layer and an optical waveguide layer in the case of the leaky mode measurement apparatus) or a diffraction grating is formed. The image is distorted because the light beam is refracted at a light emission plane of the prism-shaped dielectric block. The image is also distorted because the measurement plane is inclined with respect to the axis of the light beam.
A method for correcting a detected image by an operation based on already-known information about the distortion of the two-dimensional image detected by the two-dimensional light detection means is disclosed in Japanese Unexamined Patent Publication No. 2001-255267. In this method, the detected image is corrected so that the aspect ratio of the detected image becomes the same as that of the measurement plane. The aspect ratio of the image can be restored by using this method. However, it is impossible to recognize at which part of the measurement plane the sample is measured. Therefore, when two-dimensional physical properties of a single sample are obtained as described above, there is a problem that the two-dimensional distribution of the physical properties is erroneously obtained. Further, when the property of each of a plurality of samples which are two-dimensionally arranged on the measurement plane is obtained, there is a problem that the physical property of a certain sample is erroneously obtained as that of a different sample.